1. Field of the Invention
The present invention generally concerns the manufacture of optical components of all kinds by molding them from a synthetic material.
It is more particularly, but not exclusively, concerned with ophthalmic lenses, more precisely with ophthalmic lenses of negative power having a relatively thick edge, manufactured from a thermoplastic synthetic material, such as polycarbonate, for example.
2. Description of the Prior Art
As is known, one of the general problems to be overcome in molding a synthetic material is that it is necessary to compensate for the shrinkage which occurs on setting, however minimal this may be.
This applies to polycarbonates, even though the shrinkage is in this case less than 1%.
Traditionally, ophthalmic lenses of thermoplastic synthetic material are injection molded, involving the introduction of a thermoplastic synthetic material into an appropriate molding cavity under pressure.
However, apart from the problem of shrinkage on setting inherent to this thermoplastic synthetic material, already mentioned above, and of itself sufficient to result in diverse imperfections of the molded ophthalmic lenses, especially surface defects, there is a supplementary problem in the case, for example, of negative power opththalmic lenses with thick edges, that is to say ophthalmic lenses having a greater thickness along the edge than in the center.
On injection into the molding cavity, the synthetic material tends to flow into the section of the molding cavity of greatest thickness.
Thus beginning at any point on the periphery of the molding cavity, it progresses along this periphery, on either side of the corresponding inlet passage and in a crescent shape, until it closes on itself at the center of the molding cavity when the two arms of this crescent shape join up.
The inevitable result at the corresponding junction is the formation of a substantially radial weld line which, visible on the lens thus obtained, in practice renders it unusable.
To overcome these difficulties, it has already been proposed, inter alia in French Pat. No. 2 380 117 (Application No. 77 03937), to use a molding device comprising, to define a molding cavity, a vertical axis cylindrical sleeve and two molding dies, one concave and the other convex, which shut off the cylindrical sleeve transversely, which are axially spaced from one another and at least one of which, the concave molding die, is mounted in said sleeve so as to be slidable between an initial waiting position and a final molding position, said sleeve having a transverse opening forming an inlet passage adapted for the introduction of synthetic material into the molding cavity and said concave molding die being, in its initial waiting position, beyond the inlet passage relative to the convex molding die, a relief chamber being in communication with said molding cavity and consisting, in practice, of two pockets disposed laterally and each connected to the molding cavity by a separate passage.
Following injection of the synthetic material into the molding cavity under pressure, the concave molding die is moved in the direction towards the convex molding die, which constrains at least part of the synthetic material previously introduced into the molding cavity to be discharged in the direction towards the associated relief chamber, already charged with this synthetic material to a greater or lesser extent on its injection.
A molding device of this kind, providing effective compensation for the shrinkage of the synthetic material used on setting and which, at least in theory, and in practice within certain limits, minimizing the risk of a weld line forming on the ophthalmic lenses obtained, while offering the conjoint advantage of a reduction in the thickness of the lenses at the center, the molding chamber which it incorporates being capable of absorbing a non-negligible proportion of the quantity of synthetic material previously introduced into the molding cavity, has a number of disadvantages, however.
First of all, as the molding cavity remains in communication at all times with the inlet passage and with the relief chamber associated with it, even in the final position of the concave molding die, the lens obtained inevitably has a number of integral sprues, which must be removed and traces of which inevitably remain.
It is therefore necessary to carry out further work on the lens, which is not directly usable.
At best, the molding device in question is able to produce a negative power lens having a thickness at the center of the order of at least 3 mm which, for a typical frontal surface area, results in a very thick edge if the power of the lens is relatively high, so that the lens is virtually unusable unless its front surface area is significantly reduced. However, it is not possible with the molding device in question, by virtue of the injection process which is necessarily utilized and of the continuous nature previously emphasized of the communication between the molding cavity and its inlet passage, to obtain a lens of positive power having a thickness at the edge less than the height at least of said inlet passage, itself necessarily of consequence for injection to be carried out under proper conditions given the viscosity of the synthetic material to be injected, which can result in the lens obtained having a thickness at the center greater than necessary, at the cost of unnecessary consumption of synthetic material and reduced user comfort.
Also by virtue of the injection process necessarily utilized and the continuous nature as emphasized hereinabove of the communication between the molding cavity and the relief chamber associated with the latter, the stresses which are inevitably generated within the lens as it sets extend transversely, on the one hand between the scar corresponding to the inlet passage of the molding cavity and on the other hand the scar corresponding to the passages connecting this molding cavity to the pockets constituting the relief chamber associated with the latter.
This stress pattern, clearly visible on a device for visually displaying such forces, can result in problems regarding the homogeneity of the lens obtained, especially on subsequent treatment of the latter, such as a coloring treatment, for example.
The general objects of the present invention are a method and a device for molding optical components of any kind with which it is possible to minimize, or even eliminate, the aforementioned disadvantages and offering, among its advantages, the production of finished and directly usable optical components, such as ophthalmic lenses, for example.